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Greener Australian outback is lowering atmospheric CO2

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Plant growth in the Australian outback has taken an increasingly dominant global role in keeping atmospheric carbon dioxide levels in check, found an article published in the journal Nature.

Of more than 30 billion tonnes of carbon dioxide (CO2) emitted each year from human activity, less than half remains in the atmosphere, with the rest being absorbed by the oceans and by plants on land, called carbon sinks.

While the size of the ocean sink has remained rather steady, the amount of CO2 taken up by plants has grown over the past 30 years, including a record sink in 2011.

The latest study, published in the journal Nature, located the source of that extra plant growth, to semi-arid systems in the southern hemisphere and especially Australia.

“We find that the global carbon sink anomaly was driven by growth of semi-arid vegetation in the Southern Hemisphere, with almost 60 per cent of carbon uptake attributed to Australian ecosystems,” the study authors said in the article, titled “Contribution of semi-arid ecosystems to variability of the global carbon cycle”.

They found that increased rainfall as a result of a particular phase in the El Nino-La Nina Pacific Ocean weather cycle was partly responsible.

Green growth in the outback may be short-lived, diminishing the carbon sinks’ values (Pic: AnnieMullinsUK/Flickr)
Green growth in the outback may be short-lived, diminishing the carbon sinks’ values (Pic: AnnieMullinsUK/Flickr)

The findings suggested that such dry lands were taking over from tropical rainforests as the main land-based carbon sink.

One concern was the variability of the climate in such regions, where more rain one year could be succeeded by drought the next, undoing the extra carbon absorption.

Another concern was the fact that the plants in such regions may be rather short-lived, unlike the hardwood trees in tropical forests which have traditionally served the role of carbon sinks.

Semi-arid regions are also vulnerable to wild fire, flooding, soil erosion and over-grazing.

“More research is needed to identify to what extent the carbon stocks accumulated during wet years are vulnerable to rapid decomposition or loss through fire in subsequent years,” the article said.

Vital reserves

The study authors used a range of techniques, including a model which simulated global plant growth in response to rising CO2 levels.

Understanding better what causes the uptake of CO2 could help scientists understand atmospheric levels through the course of this century, and so better predict climate change.

Carbon sinks serve an important role to limit climate change.

Even as growth in greenhouse gas emissions has accelerated over recent decades, the fraction of CO2 that accumulates in the atmosphere (the airborne fraction) has remained largely unchanged, on average, since 1959, at 44%.

But the airborne fraction changes from year to year, varying between 18% and 79% over the past half century, the Nature article said.

The study investigated the size of the land-based carbon sink over the past three decades, with particular interest in a surge in carbon absorption in 2011.

They matched model-based simulations with the satellite record. The latter showed record “greening” in 2011 of the landscape over three Southern Hemisphere semi-arid regions – in Australia, temperate South America and southern Africa.

High rainfall, associated with the La Nina cycle of the El Nino Southern Oscillation (ENSO) was partly responsible for the 2011 spurt in plant growth, they concluded.

That recent increase in rainfall combined with a long-term semi-arid regional greening trend.

“The greening trend in semi-arid regions has been previously associated with a range of drivers that include altered precipitation frequency and intensity, increased water-use efficiency (by plants) … and woody encroachment following land-use and grazing.”

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